Etching of tantalum and columbium foils



Jan. 10, 1957 c. F. RERAT 3,297,555

ETCHING OF TANTALUM AND COLUMBIUM FOILS Filed July 24, 1964 FIG. 1

INVENTOR Carlos F. Rerof BY ZMMM/QQQM ATTEDRNEYS United States Patent 3,297,555 ETCHIN G OF TANTALUM AN D COLUMBIUM FOZLS Carlos F. Rerat, Hamburg, Pa, assignor to Kawecki Chemical Company, Boyertown, Pa., a corporation of Pennsylvania Filed July 24, 1964, Ser. No. 384,836 3 Claims. (Cl. 204-141) This invention relates to the etching of tantalum and columbium and, more particularly, to the etching of foils of these metals without producing pinholes in the foil.

In order to increase the effective surface of tantalum and colu'rnbium foils for use as electronic capacitors, it is conventional practice to immerse the foil in an etching medium such as an alcoholic solution of an inorganic halide and to effect etching of the foil surface by passing a pulsating current to this surface. Inasmuch as capacitance increases with depth of etching, it has been the aim to obtain as much etching as possible without producing pinholes. However, it has been found in practice that the relatively high current densities which produce the desired degree of etch also promote formation of pinholes and render the foil worthless for capacitor use.

As a result of extensive investigation, Ihave found that pinholing appears to result from incomplete dissolution of the etching product during the off period of each cycle, the residual etching product causing the on portion of the current cycle to concentrate in the areas of the foil surface where the etching product is most nearly or completely removed after the on cycle. As a con sequence of this concentration of the duty cycle at certain areas, an exceptionally high etching current density is developed at these areas and rapidly erodes through the thin foil in the form of pinholes. I have discovered that pinholing can be avoided during electrolytic etching of tantalum and columbium foil by control of the duty cycle and frequency of the pulsating etching current. Control of the duty cycle (the ratio of the on period to the complete cycle) establishes control over the relative proportions of the on and off periods, and control of the frequency establishes control over the duration of the off period. Thus, the present improvement in the electrolytic etching of tantalum and coiumbium foil comprises adjusting the average current of the pulsating duty cycle to effect the desired amount of etching, and adjusting the duty ratio and frequency of the pulsating current to a value such that the off portion of the total cycle period is at least equal to the time required for the product of the etching current to be dissolved by the etching medium. In order to obtain maximum etching efficiency, the frequency of the duty cycle is further adjusted so that the ofiF portion of the total cycle period does not substantially exceed the time required for the product of the etching current to be dissolved by the etching medium.

These and other novel features of the invention will be more readily understood by reference to the accompanying drawing in which:

FIG. 1 is a graph of pulsed etching current versus time in which the on portion of the duty cycle equals the solution time for the etch product;

FIG. 2 is a similar graph for the same frequency and same average pulsed current in which the on portion of the duty cycle has been shortened and the solution time for the etch product has been lengthened;

FIG. 3 is a similar graph in which the frequency of the pulsed etching current of FIG. 2 has been decreased so as to obtain the same length of on portion of the duty cycle as in FIG. 2 but an extended solution time for the etch product; and

Fatented Jan. 10, 1967 FIG. 4 is a similar graph in which the same duration of solution time for the etch product is obtained as in FIG. 3 by using an increased peak current, a shortened on period and the same frequency as in FIGS. 1 and 2.

The practice of the invention can be illustrated by reference to the four graphs of the drawing. In each of these graphs the vertical coordinate represents the value of etching current and the horizontal coordinate represents time. The heavy vertical lines in each graph represent also the beginning of successive cycles, the length of the etching current cycle in each figure being indicated by the time interval t In each figure the value of the pulsed etching current (i,) is such with respect to the length of the on period Of each cycle as to produce the same average etching current (i indicated by the horizontal broken line. Inasmuch as the product of etching current and time on is a measure of the amount of electricity used for etching in each cycle, this amount (represented by the shaded area in each figure) also is a measure of the amount of etching product produced during the on portion of each cycle. The time available for this etching product to be dissolved by the etching medium during the off portion of each cycle is indicated by the bracketed portion z (time of solution) along each horizontal time coordinate. In each graph it is assumed that the potential during the off portion of each cycle is sufficiently negative to overcome the back and thus produce a I period of zero current during which dissolution of the etching product is free to proceed.

In using the aforementioned graphs to illustrate the practice of the invention, a value of average current has been chosen which, based upon electrochemical computations, is such as to produce the desired amount of etching of the metal foil.

The situation represented by FIG. 1 corresponds to that of a conventional sinusoidal electrolyzing current in which the on period is equal to the off period. If the value of the electrolyzing current is such that there is not sufficient off time to dissolve the etch product before the beginning of a succeeding on period, the etching current concentrates in any portion of the foil surface where the layer of etch product is the thinnest or has already been dissolved by the etching medium. I have found that this concentration of current, expressed more conventionally as a locally increased etching current density, promotes local catastrophic etching and results in the development of pinholes before a significant overall etching of the foil surface to the desired extent is achieved.

The situation represented by FIG. 2 corresponds to a modification of the operation represented by FIG. 1 in which, pursuant to the present invention, the duty cycle (the ratio of on period to total cycle time W is decreased in order to increase the length of the off period during which dissolution of the etch product takes place. It will be observed that the average current value is maintained by increasing the magnitude of the current pulse while shortening its duration. If the length of the off period (1 is not sufiicient to permit adequate dissolution of the etch product and the pulse generator is incapable of providing a stronger current pulse, the off period can be lengthened to the necessary extent, as shown in FIG. 3, by decreasing the frequency of the pulse and thus increasing the cycle time (1 However, if the generator design so permits, the magnitude of the pulse can be increased, as shown in FIG. 4, so as to obtain, by a shortening of the pulse duration (t the same lengthening of the off period.

It will be seen, accordingly, that by using a pulsating current of sufiicient magnitude to produce the desired amount of etch, control of the etching operation to prevent formation of pinholes is effected pursuant to the invention by adjusting the duty ratio and frequency of the pulsating current to a value such that the o portion of the total cycle period is at least equal to the time required for the product of the etching current to be dissolved by the etching medium. An excessively long ofF portion of each cycle represents a period during which no part of the etching process is taking place and is a wasted portion of etching current cycle. It will be readily apparent, however, that this wasted portion of each cycle can be eliminated by decreasing the cycle time (t that is, by increasing the frequency of the pulsating current. Pursuant to the practice of the invention, the frequency is increased up to, but not beyond, that value which results in a cycle time equal to the length of the on period plus the requisite solution period. With such an increase in etching cycle frequency, more on periods are obtained over any length of etching time and thus more etching is obtained while avoiding the condition which would promote pinholing.

In each of FIGS. 1 through 4 it will be observed that the lower end of the shaded etching area adjoins a clear area. The clear area corresponds to the ineffective exchange current (i portion of each duty cycle when the value of the current is below that at which etching is effected. Accordingly, in the interest of efficiency, it is desirable to operate with a relatively high current value so as to minimize the effect of the exchange current.

From the foregoing discussion it is clear that the most etficient practice of the present invention is obtained by making the etching current as high and the duty cycle as low as is possible within the limitations of the equipment used. Current pulse generators are presently commercially available in which the value of the current, the duty cycle and the frequency are adjustable, and the choice of such equipment for the practice of the invention is dictated by the values of these variables applicable to tantalum and columbium foils according to presently known etching parameters. For example, it is already well established in the art what etching currents (with conventional duty cycles) will produce a desired amount of etch. The rate of dissolution of the etch product in various etching media are also known or can be readily determined. From these values, pulse generator equipment can be easily evaluated so as to obtain the optimum pulse current, duty cycle and frequency range to adapt the equipment to the practice of this invention.

The following examples show the difference in result obtained by the practice of the invention over the prior art practices:

Example I Pieces of tantalum foil 0.0005 inch thick and 4.25 square inches in area were electrolytically etched in a standard etching electrolyte comprising an alcoholic solution of an inorganic halide. The foil samples were etched pursuant to conventional practice using rectified 60 cycle alternating current of standard sinusoidal wave form, each etching test being carried out for 12 minutes at a meter or average current (using a DC. meter) such as to provide the specified current density at the foil surface from which the total number of ampere minutes (identified as etching density) was computed. The etched foil samples were then formed under conventional current conditions in a 0.01% phosphoric acid solution maintained at 90 C.:3 C. The capacity of the formed samples was measured at 20 volts and at 67 volts D.C., after being held at the stated terminal voltage for 30 minutes. The results of these tests are reported in the following table where, in addition, there is further given the weight loss produced by the etching operation and the physical .character of the etched foil as determined by light transmission and expressed in terms of the having pinholes:

area of the foil Samples of the same foil of the same size were then etched pursuant to the present invention using current pulses at a frequency of 30 pulses per second with a duty ratio of 46% (i.e., 18 milliseconds off per cycle), these values having been determined by chemical and electrochemical calculations to provide an off portion of each cycle at least equal to the time required for the product of the etching current to be dissolved by the etch medium. Each sample was etched at the same meter current rate of 1 ampere for varying lengths of time in order to obtain various etching densities for comparison with the data in the preceding table. All other conditions and tests were as before. The results of these tests areas Example II The same comparison as in Example I was made using a different source of tantalum foil but dipulicating all other conditions of Example I. Test series A were run with the aforementioned conventional etching procedure and test series B" were run using the etching procedure of the invention as described in Example I:

TABLE III Current Etching Sample Physical Density, Density, Weight Capacity, ii/in; C har- Amp/in. Arno, Loss, gin. at 20 v. at 67 v. acter min/in.

A0088 1. 055 0. 0930 31.1 6. N I II 0.176 -s 2. 110 0. 1925 43. 0 E). 20 50% PH 0.264 3.180 0. 2610 37. 6 8. P [1 Current Etching Sample Physical Density, Density, Weight Capacity. ut/in. Char- Amp/in. Arno, Loss, gm. at 20 v. at 67 v. aetcr llllIL/in 1 130.235 0. 940 0. 0957 27. 3 6. 14. N PH 1. 410 0.1310 3'2. 5 6. 00 N PH 1. 880 0.1700 40. 0 9. 87 N P11 2. 350 0. 2160 53. 0 11.08 3% P H 2. S20 0. 2457 63. 2 12. 70 N PH 3. 290 0. 3035 73. 5 14. 35 N III In the results summarized in the tables of Examples I and II, it will be readily understood that the measured weight loss is an indication of the severity of the etching. The data show that with conventional etching procedure only a relatively small weight loss can be tolerated before pinholing develops. In the etching procedure of the present invention, however, much more extensive etching can be achieved without pinholing, and this increased etching results in capacities several times as great as those that could be achieved by the conventional etching procedure.

Within the prescription that the duty ratio and fre quency of the pulsating etching current be adjusted to a value such that the off portion of the total cycle is at least equal to the time required for the product of the etching current to be dissolved by the etching medium, it has been further discovered that the choice of duty ratio and frequency affects the capacity of the tantalum foil. This relationship is demonstrated by the data in the following table obtained for identical samples of 0.0005 inch thick tantalum foil, each 4.25 square inches in area. Each sample was etched with a meter current of 0.590 milliampere for a uniform period of 12 minutes, and was formed and tested as described hereinbefore.

TABLE IV Duty Ratio, Frequency, Capacity, ,uf/iu. at Sample Percent p.p.s. at 20 v. 200 v. Weight Loss, gm.

The weight loss for each sample was substantially uniform, indicating a uniform degree of etch in each sample.

However, the capacity of the samples varied considerably with changes in duty ratio and pulse frequency. From this data it will be readily apparent that a wide variety of capacity values can be obtained by control of duty ratio and frequency and, furthermore, that if some predetermined capacity is desired in the etched foil the use of lower frequencies promotes greater control in achieving this desired capacity in a production operation.

I claim:

1. In the method of etching tantalum and columbium foil wherein the foil is immersed in a predominantly alcoholic electrolytic etching medium including at least one inorganic halide and a pulsating duty-cycle current having an o period between each pulse wherein substantially no current flows is passed to the surface of the foil to effect the desired etching thereof, the improvement which comprises adjusting the average current of the duty cycle to effect the desired amount of etching, and adjusting the duty ratio and frequency of the pulsating current to a value such that the off portion of the total cycle period is at least equal to the time required for the product of the etching current to be dissolved by the etching medium.

2. The method according to claim 1 in which the frequency of the duty cycle is so adjusted that the 011 portion of the total cycle period does not substantially exceed the time required for the product of the etching current to be dissolved by the etching medium.

3. The method according to claim 1 in which the average etching current is maintained as high as possible so as to minimize the effect of exchange current.

References Cited by the Examiner UNITED STATES PATENTS 2,742,416 4/ 1956 Jenny 204-141 2,863,811 12/1958 Ruscetta 204-141 2,930,741 3/1960 Burger et a1. 204-141 3,070,522 12/1962 Robinson et a1 204-141 3,190,822 6/1965 Burnham 204-141 FOREIGN PATENTS 618,931 3/ 1949 Great Britain.

856,927 12/ 1960 Great Britain.

923,783 4/ 1963 Great Britain.

JOHN H. MACK, Primary Examiner.

R. K. MIHALEK, Assistant Examiner. 

1. IN THE METHOD OF ETCHING TANTALUM AND COLUMBIUM FOIL WHEREIN THE FOIL IS IMMERSED IN A PREDOMINANTLY ALCOHOLIC ELECTROLYTIC ETCHING MEDIUM INCLUDING AT LEAST ONE INORGANIC HALIDE AND A PULSATING DUTY-CYCLE CURRENT HAVING AN "OFF" PERIOD BETWEEN EACH PULSE WHEREIN SUBSTANTIALLY NO CURRENT FLOWS IS PASSED TO THE SURFACE OF THE FOIL TO EFFECT THE DESIRED ETCHING THEREOF, THE IMPROVEMENT WHICH COMPRISES ADJUSTING THE AVERAG CURRENT OF THE DUTY CYCLE TO EFFECT THE DESIRED AMOUNT OF ETCHING, AND ADJUSTING THE DUTY RATIO AND FREQUENCY OF THE PULSATING CURRENT TO A VALUE SUCH THAT THE "OFF" PORTION OF THE TOTAL CYCLE PERIOD IS AT LEAST EQUAL TO THE TIME REQUIRED FOR THE PRODUCT OF THE ETCHING CURRENT TO BE DISSOLVED BY THE ETCHING MEDIUM. 